The invention relates in general to antenna structures in radio apparatus. In particular the invention relates to an antenna structure which has two resonating frequencies different from each other. This patent application uses a mobile phone as an example of a radio apparatus.
In different parts of the world there are cellular radio systems in use that differ from each other significantly in their operating frequency ranges. As regards digital cellular radio systems, the operating frequencies of the Global System for Mobile Telecommunications (GSM) are in the 890-960 MHz range, the operating frequencies of the Japanese Digital Cellular (JDC) system are in the 800 MHz and 1500 MHz bands, the operating frequencies of the Personal Communication Network (PCN) are in the 1710-1880 MHz range, and those of the Personal Communication System (PCS) in the 1850-1990 MHz range. The operating frequencies of the American AMPS mobile phone system are between 824 MHz and 894 MHz and those of the Digital European Cordless Telephone (DECT) system in the 1880-1900 MHz range.
Since the resonating frequency of a prior-art radio-frequency antenna depends in a known manner on the length of the antenna, through the wavelength, a particular antenna can be used only in a mobile phone designed for a single-frequency cellular radio system. In some cases, however, it is desirable that one and the same phone could be used in some other frequency range, too. In addition to other suitable RF parts, a working antenna arrangement is then needed.
U.S. Pat. No. 4,442,438 discloses an antenna structure resonating at two frequencies, comprising, as shown in FIG. 1, two helices 101, 102 and one whip element 103. The helices 101 and 102 are positioned one after the other and their adjacent ends 104 and 105 constitute the feed point of the combined structure. The whip element 103 is partly inside the upper helix 101 and its feed point 106 is at its lower end. An RF signal is brought to the feed point 106 via a coaxial conductor 107 coinciding with the symmetry axis of the structure and traveling through the lower helix 102. The feed point 106 of the whip element is coupled to the lower end 104 of the upper helix, and the lower helix is coupled at its upper end 105 to the conductive and grounded shroud of the coaxial conductor 107. The structure""s first resonating frequency is the resonating frequency of the combined structure of helices 101 and 102; 827 MHz in the illustrative embodiment. The second resonating frequency of the structure is the common resonating frequency of the upper helix 101 and the whip element 103; 850 MHz in the illustrative embodiment. Thus, helix 101 and whip element 103 are such that they have substantially the same resonating frequency.
The structure disclosed by the US patent is relatively complex. From the manufacturing standpoint, the most difficult part in the structure is the feed point arrangement at the middle of the antenna, where the lower end 106 of the whip element and the lower end 104 of the upper helix have to be galvanically coupled, and the lower helix has to be coupled at its upper end 105 to the shroud of the coaxial conductor feeding the whip element. According to the material presented in the patent the difference between the two resonating frequencies achieved by the structure is small because the dimensions of the upper helix 101 and the whip element 103 have to be such that they have substantially the same common resonating frequency, so the structure cannot be applied to a phone operating at the GSM and PCN frequencies, for example. Indeed, in the description of the patent it is stated that an object of the invention is to broaden the resonating frequency area of the mobile phone antenna such that it would better cover the whole frequency range in one cellular radio system.
FI patent application 963275 (LK-Products) discloses a dual-frequency antenna structure according to FIG. 2 in which there is at a certain point between the ends of a helix antenna 201 wound into a cylindrical coil a coupling part 202 for coupling to a second antenna element 203. The cylindrical coil conductor 201, which is the first antenna element in the antenna, comprises in the direction of its longitudinal axis a lower part 204 and an upper part 205, and the second antenna element 203 is connected to the cylindrical coil conductor through a fixed coupling at the coupling point 202 between the lower and upper parts. The two radiating antenna elements of the structure have a common lower part up to the branching point consisting of the coupling part, from which point on the electrical lengths of the antenna elements are different. The first resonating frequency of the combined antenna structure is determined by the total electrical length of the common lower part of the antenna elements and the upper part of the first antenna element. The second resonating frequency is determined by the total electrical length of the common lower part of the antenna elements and the upper part of the second antenna element. In addition, the resonating frequencies are affected by the mutual coupling of the antenna elements and the fact that the antenna elements are electrically conductive bodies in the near fields of one another so that they put a load on each other. The antenna structure according to FIG. 2 is relatively difficult to precisely dimension to the desired frequencies since the coupling point between the antenna elements requires quite accurate positioning. In addition, the electrical coupling in the coupling point easily becomes unreliable.
FI patent application 970297 (LK-Products) discloses an antenna according to the principle illustrated in FIG. 3 wherein an antenna element 301 has a first end and a second end and a tapping point 302 which is located at a certain point between the ends of the antenna element. The tapping point divides the antenna element asymmetrically such that the electrical length from the tapping point to the upper end is considerably greater than the electrical length from the tapping point to the lower end. The feed conductor 303 of the antenna, which connects the antenna element electrically to a radio apparatus, is coupled to the antenna element at the tapping point. A substantial portion of the feed conductor also serves as a radiating element because the feed conductor is electrically unshielded, i.e. it has no shroud made of a conductive material around it. The total electrical length of the antenna structure at a first operating frequency is the sum of the electrical lengths of the feed conductor 303 and the portion extending from the tapping point 302 to a first end of the antenna element 301. Correspondingly, the total electrical length of the antenna structure at a second operating frequency is the sum of the electrical lengths of the feed conductor 303 and the portion extending from the tapping point 302 to a second end of the antenna element 301. The antenna element 301 may be a helix, a straight conductor or a combination of those. The disadvantage of this antenna structure is the difficulty in manufacturing the antenna structure such that the tapping point 302 will be sturdy.
An object of the present invention is to provide an antenna structure which can be applied in two operating frequency ranges and which is simple to manufacture and reliable in its operation. Another object of the invention is to provide an antenna structure which can be easily dimensioned to two different operating frequencies. A further object of the invention is that the antenna structure according to the invention is applicable to large-scale series production.
The objects of the invention are achieved by using as an antenna element a helix the pitch of which decreases when moving away from the feed point.
The antenna according to the invention comprises a cylindrical coil conductor having a turn A and turn B and other turns between them. The antenna is characterized in that the pitch of turn A does not equal the pitch of turn B and the pitches of the other turns between turn A and turn B are arranged according to the magnitude between the pitch of turn A and the pitch of turn B.
It is known that a conductive body may have multiple resonating frequencies the lowest one of which is the so-called fundamental frequency, the rest being harmonic frequencies. The invention is based on the observation that the resonating frequency of a cylindrical coil conductor, or helix, is changed when the dimensional parameters of the helix are changed in the various parts of the structure. The electrical length of the helix conductor determines the fundamental frequency. In connection with helices, the distance between the ends of a turn in the direction of the longitudinal axis of the helix is called a pitch. When the feed point is at one end of a helix and the pitch either decreases or increases towards the other end, the mutual interaction of the turns changes the resonating frequencies. When the number of turns, pitch of the helix at various points and other parameters are suitably selected, the resonating frequencies will be at such positions on the frequency axis that the structure can be used in two cellular radio system frequency ranges.